Automatic control device



April 16, 1935. D. J. STEWART AUTOMATIC CONTROL DEVICE Original FiledMay 16, 1930 INVENTOR Barman cl Stewar I5,

ATTORNEYS Patented Apr.v 16, 1935 PATE Prior.

AUTOMATHC CONTRGL DEVECIE Duncan'J. Stewart, Rockford, Ill, assignor toHoward D. (Jolman, Rockford, Ill.

Application May 16, 1930, Serial No. 452,911 Renewed November 27, 1933 9Claims. (Cl. 20o i39) This invention relates to sensitive automaticcontrol devices such as humidostats and thermostats and has moreparticular reference to devices for controlling electric circuits.

In devices of the above character, the motion of the sensitive controlelement resulting from changes in temperature or other conditions of thesurrounding air is applied to a control switch through the medium of amember which is made relatively long and-of light construction in orderto properly augment the motion of the sensitive element and cause theswitch to be opened or closed with the required accuracy. In view ofthis necessarily delicate and flexible mounting, jarring of the supportsfor the instrument or vibration thereof such as isoften encountered inservice will produce sufiicient movement of the switch actuating memberto open and close the switch against the action of the sensitive elementand at a rapidly recurring rate. As a result, objectionable andprolonged sparking frequently takes place at the contact points whichbecome burned off and disfigured to such an extent as to impair thesensitivity of the device. Such sparking is also objectionable in thatit seriously inter-' feres with the operation of radio the vicinity ofthe instrument.

-The primary object of the present invention is to provide an automaticswitch controlling instrument such for example as a thermostat having amagnetic detent associated therewith in a novel manner such as toproduce a quick and positive closure of the controlling switch withoutmaterially reducing the sensitivity of the instrument as a whole.

The invention also resides in the novel and compact manner in which themagnet is associated with the control instrument.

I Other objects and advantages of the invention will become apparentfrom the following detailed description taken in connection with theaccompanying drawing, in which Figure 1 is a front elevational view of athermostat embodying the features of the present invention. the coverbeing broken away to show the working parts.

Figs. 2 and 3 are sectional views taken respectively along the lines 2-2and 3-3 of Fig. 1.

Fig. 4 is an enlarged fragmentary view of certain of the magnetic parts.

While the invention is susceptible of various receiving sets inmodifications and alternative constructions, I

have shown in the drawing and will herein describe in detail thepreferred embodiment, but it is to be understood that I do not therebyintend to limit the invention to the specific form disclosed, but intendto cover all modifications and alternative constructions falling withinthe spirit and scope of the invention as expressed in the appendedclaims,

In the form shown in the drawing,.the inven= tion is embodied for thepurpose of illustration in a thermostat, the operating parts of whichare housed within a casing formed by a flat elongated base 5 and aremovable perforated cover 6 therefor. The sensitive element, which isadapted to respond to temperature changes in the surrounding air, is inthe form of a coiled bimetallic strip 7 adjustably supported at thelower end of the base in a manner well knownin the art. For thispurpose, one end of the strip is secured to a bell-crank 8 pivoted on astud 9 and having an arm l connected through the medium of an eccentricscrew l to a lever l2 also fulcrumed on the stud 9 and extendingupwardly along the base 5. At its upper end, the lever i2 is connectedto a lever l3 adjacent the'latters fulcrum which is formed by a stud Hi.The lower end of the second lever projects through a slot l5 in thecover 6 and may be swung in one direction or the other to shift thesupported end of the strip 1 and thereby change the setting of thethermostat.

Lateral fiexure of the free end of the thermostatic strip 1 in responseto temperature changes in the surrounding air operates a switch.actuating element in the form of an elongated tongue l6 rigidly attachedto the free end of the strip.

The tongue extends longitudinally of the base 5 and carries at its upperend an electrical contact I! which cooperates with two adjustablecontacts Hi to form control switches l9 and The contacts l8 are arrangedin opposed relation on opposite sides of the tongue contact and arecarried by adjusting screws 2| threading through studs 22 whichconstitute binding terminals for the two switches. For thermostats ofthischaracter, the contacts 18 are spaced to give the common contact lla total range of movement of approximately 0.002 inch. While it isdesirable in order to obtain optimum sensitivity to maintain such asmall range of movement of the switch arm l1, this range of movement maybe increased slightly without danger of increasing the sensitivity' ofthe instrument to an objectionable degree. Such increase, for example,may occur by reason of deterioration or burning off of the contactsurfaces in service use. With the parts thus arranged, it will beapparent that one control switch will be opened and the other closedwhen the temperature rises above or falls below the value predeterminedby the combined settings of the screw H and the adjusting lever l3.

The present invention contemplates the use, in a control device of thegeneral character above described, of a single permanent magnet havingits opposite poles disposed on opposite sides of an armature which moveswith the switch actuating member or tongue IS. The armature is formedherein by the upper end portion 23 of the tongue which, for thispurpose, is composed inch. Therefore, as shown in Figs. 1 and 3, the

contact points 18 constitute stops for preventing the armature 23 fromcoming in contact with or approaching very near to the pole faces. Thearmature is thus confined to a narrow range of movement which, in thepresent instance, is 0.002 of an inch and occupies the central zone ofthe gap between the pole faces.

The remote ends of the pole pieces are joined by a magnetic member inthe form of a U-shaped yoke 25 composed of soft readily workablemagnetic iron and having its flat intermediate portion secured againstthe base 5 immediately below the contacts l8. The parallel legs 26 ofthe yoke proje'ctforwardly from the base on opposite sides of thearmature and are formed with apertures into which the remote ends of thepole pieces are pressed and thereby firmly supported. By em 'ploying aseparately formed yoke for connecting and supporting the pole pieces 26,the latter may be formed conveniently in the desired cylindrical shapeand composed of cobalt steel which will retain a high degree ofpermanent magnetism but cannot be machined readily or otherwise workedinto the shape required in the present instance in order to aline thepole projections in end-to-end relation on opposite sidesof thearmature;

Subsequent to the assembly of the pieces 2% in the yoke, the magnet thusformed is polarized in a well-known manner whereupon the adja cent endsof the pieces become poles of opposite polarity and constitute sourcesof permanent magneto-motive forces which cause a magnetic flux to threadthe gap between the two pole faces and a highly permeable return path.As shown in Fig. 3, this return path-is spaced from the armature 23 when,the magnet is mounted as above described and extends through the yoke25 between the outer end ofthe pole pieces. Thus the magnetic fluxthreads the armature in a direction substantially perpendicular theretoasis shown by the arrows in Fig. 4.. Since both pieces 24 constituteparts of the same magnetic circuit,

they act cumulatively in contributing to the total magneto-motive forceregardless of the fact that they may be polarized to different degrees.Be-

cause of the low. reluctance of the magnetic circuit as a whole, astrong magnetic field is produced with a relatively small magnet.

The armature 23, it will be observed, is flexibly supported by the strip1 and tongue NS for broadwise movement longitudinally of the gap in themagnetic circuit and therefore is under the influence of two oppositelyacting forces which are due to the respective poles and which vary inmagnitude according to the spacing of the armature relative to thepoles. Thus, when the armature is displaced to one side of the magneticcenter of the gap, the attractive force of the nearer pole predominatesand the resultant force firmly maintains the armature in a limitingposition spaced from the pole .and determined by theassociatedstationary contact l8. This force decreases as. the armature moves toits other limiting position and as the-armature passes beyond themagnetic center, the oppositely acting force due to the other polebecomes the predominating force and serves to hold the tongue againstthe other stationary contact thereby effectually maintaining the otherswitch closed.

The strip 1 permits or resiliently resists movement of the armaturebythe resultant of the two attractive forces depending on the existingtemperature. Thus, as the temperature change reverses after movement ofthe tongue against one stop l8, the strip becomes strained, the en ergyat first stored being insufficient to move the tongue but eventuallyincreasing with the temperature change to a value sufiicient to overcomethe magnetic force tending to hold the switch closed. When theattractive force is finally overcome, the tongue moves quickly and withasnap action to its opposite limiting position.

As a result'of this snap action, the opening and closure of both controlswitches is made to occur positively and accurately and with a slightblow at the contact points which 'are thereby cleared of any detrimentalparticles of foreign material. The contacts of the closed switch arealways drawn together firmly under sufiicient pressure, due to thepredominating magnetic influence, to prevent their separation as anincident to jarring or other vibration of the thermostat. Moreover, whenthe switch opening movement of the tongue has been initiated by thebuilding up of the required amount of energy in the resilient strip 1,.completion of the switch opening is completed rapidly and reclosure ofthe switch cannot take place. Sparking at the contact points and theconsequent deterioration thereof are efiectually minimized.

From the foregoing considerations it will be apparent that the presentsthermostat is well adapted for the control of oil burners and the likewhere only one switch is required and this must be operated to start theburner at a temperature slightly lower than that at which it is operatedto stop the burner. With the present thermostat this differentialtemperature range may be varied as desired by changing the strength ofthe magnet or the position of the tongue stops.

The desirable snap action in the movement of the thermostatic tonguebetween its limiting positions as above-described is obtained with thepresent magnetic structure by taking advantage of the fact that the twoopposing magnetic forces acting on the tongue armature change at a morerapid rate due to movement of the armature than does the tension on thethermostatic strip. Thus, according to a well known law, the attractiveforce exerted on the armature by each pole is equal to f (B cos x) da,

'the attractive force due to the nearer pole is greater than for thefarther pole and the average value of the flux density B is also greaterfor the nearer pole.

The other pole is spaced substantially farther from the armature whichallows the magnetic lines greater opportunity to spread out and takelonger paths rather than to create high flux densities in the short pathdirectly between the pole and armature. Such spreading action isillustrated on an exaggerated scale in Fig. 4 and causes the averagevalue of the flux density B to be diminished with displacement of thearmature away from the pole. Since this quantity is squared in the aboveequation, it will be apparent that a relatively large change in theattractive force exerted by either pole will result from a slightdisplacement of the armature. The decrease in the average value ofcosine a: as the armature moves away from the pole causes a furtherdecrease in the attractive force. Moreover, the force due to one poledecreases as the other increases, thereby further multiplying the changein the resultant magnetic force for a given displacement of thearmature. On the other hand, the change in the tension of thethermostatic strip is more nearly proportional to the displacement ofthe armature. From this it follows that whenever enough energy has beenstored in the strip 1, due to temperature changes, to initiate movementof the tongue l6 away from its engaged contact, the movement willcontinue and the tongue will be carried throughout its entire range ofmovement into engagement with the other contact.

By providing two magnetic poles on opposite sides of the armature, astrong magnet and therefore a rapid change in the magnetic force for asmall armature displacement may be provided for without placing thethermostatic strip under such strain as to impair the sensitivity andaccuracy of the thermostat. Thus it will be apparent that the oppositelyacting forces due to the magnet, tend to neutralize each other with theresult that the thermostatic strip is not under any stress due to thepresence of the magnet when the armature is on the magnet center. Thiscondition substantially prevails through the narrow range of movement ofthe armature with the result that the stresses set up in the strip aredue practically entirely to temperature changes.

The desirable snap action above described within the extremely narrowrange of the tongue movement provided for and effective equalization ofsuch action for both directions of movement of the tongue is renderedpossible, in the present instance, by constituting the opposite magneticpoles parts of the same magnetic circuit. Since the flux threading eachpole is derived from the same source of permanent magnetism, that is,the pieces 24, the poles may be made of equal strength simply byconstructing the pole faces of equal area. With the poles of equalstrength, the magnetic center of the gap between them coincides with thespace center of the gap which greatly facilitates setting of thethermostat so that the opposing pole actions will be substantiallybalanced. Such setting may be effected simply and conveniently bylocating the magnet armature on the space center of the gap when thearmature is disposed on the space center between the two limit stopsformed by the contact points Ill.

The initial or factory adjustment of the thermostat is effected asfollowsrFirst the tongue armature is located exactly midway between thepole faces by inserting plates of equal thickness in the two air gapsbetween the armature and the poles. Then the limit stops for thearmature are set by turning the screws 2| so that the contact end ofeach is accurately spaced 0.001 inch from the opposing surface of thetongue contact. -A 0.002 inch range of movement is thereby establishedfor the armature and this range is located centrally of the gap betweenthe poles. After removal of the gage plates, the screw H is adjusted sothat the tongue will respond to the prevailing temperature when theadjusting lever is set for such temperature.

The magnet constructed as above described, renders the factoryadjustment of the thermostat simple and reliable and forms anexceedingly compact structure which can be mounted within the ordinarythermostat casing without enlargement thereon.

With the magnet constructed and'the limit stops I8 and the pole facesspaced relative to the armature in the manner above described, thedesired snap or detent action is obtained without materially reducingthe sensitivity of the instrument. This result is obtained by the novelarrangement of the magnet, the limit stops I 8, the contact arm and themagnet armature whereby the attractive forces acting on the armature arerelatively weak but the rate of change of these forces by displacementof the armature is so great as to produce the desired detent action,even though the range of movement of the armature is very narrow. Theuse of a strong magnet producing relatively weak attractive forces whichchange in magnitude at a rapid rate during displacement of the armatureis made possible by locating the magnet poles on opposite sides of thearmature and spaced a substantially greater distance from the armaturethan the total range of movement of the latter. This rate of change ofthe attractive forces is increased to a maximum by positioning the limitstops and pole faces relative to the armature so that when the latter isdisposed on the magnetic center of the gap between the poles, thecontact arm will be disposed midway between the limit stops.

I claim as my invention:

1. In-an automatic control device, the combination of a base plate, anelongated laterally movable switch actuating element movably mounted onsaid base plate and extending parallel thereto, said element having amagnetic armature movable therewith, a permanent magnet secured to saidbase plate and comprising a bar extending transversely of said elementbetween the element and said base plate, legs projecting from oppositeends of said bar substantially perpendicular to said base plate anddisposed on opposite sides of said armature and alined arms projectingtoward each other from the projecting ends of said legs and definingpole faces on opposite sides of the armature.

2. In an automatic control device, the combination of a base plate, anelongated substantially straight tongue of magnetic iron constituting anarmature, means associated with one end of said tongue and supportingthe same on said base plate in parallel relation thereto, a stationarycontact cooperating with the opposite end of said tongue to form aswitch, and a permanent magnet comprising a bar supported from said baseand extending transversely of said tongue adjacent said contact, legsprojecting from opposite ends of the bar substantially perpendicular tosaid base plate, and arms extending at right angles to the projectingends of said legs and defining poles having faces disposed on oppositesides of said tongue.

3. In an automatic control device such as a thermostat, the combinationof a base, an elongated laterally movable switch-actuating elementextending along the base and providing a flat magnetic armature adjacentits free end, a pair of stops disposed on opposite sides of said elementadjacent said free end and acting to limit the movement of the end inopposite directions, a pair of longitudinally alined magnetic polesadjacent said stops providing two opposed parallel faces of equal areadisposed on opposite sides of said armature and spaced apart a distancegreater than said stops, a member of magnetic material connecting saidpoles at points spaced from said faces and extending crosswise of saidelement adjacent said stops, the magnet formed by said poles and saidmember being supported by said base.

4. In an automatic electric control device of the general characterdescribed, the combination of a substantially rigid arm supported fromone end for lateral movement of the opposite end, a sensitive controlelement responsive to changes in the condition of the surrounding airand arranged to actuate said arm, a pair of closely spacedstationary'stops engageable with said arm to limit the movement of thearm in opposite directions, said arm having rigid therewith a contactengageable directly with one of said stops to form an electric controlswitch and also a magnet armature with oppositely facing surfaces, and apermanent magnet having poles disposed on opposite sides of saidarmature with end faces opposing said armature surfaces and each spacedfrom the adjacent armature surface a distance substantially greater thanthe range of movement of the arm whereby said pole faces will be spacedsubstantial distances from said armature surfaces inithe limit-positionsof said arms.

5. In the automatic electric control device of the general characterdescribed, the combination'of a substantially rigid arm supported forlateral movement of one of its ends, a sensitive control elementarranged to actuate said arm, a pair of stationary stops engageable withsaid arm to limit the movement of the arm in opposite directions to arange of a few thousandths of an inch, said am having a contact thereonengageable directly with one of said stops to form an electric contrelswitch and having rigid therewith a magnet armature with oppositelyfacing magnetic surfaces, and a permanent magnet having poles disposedon opposite sides of said armature with end faces each spaced from theopposed armature surface a distance at least ten times as great as thespacing of the corresponding stop and said arm in either limit positionof the arm.

6. An automatic control device having, in combination, an arm, meansresponsive to changes in the condition of the surrounding air for movingthe arm laterally back and forth, a pair of closely spaced stationarystops engageable with said arm and acting to limit the movement thereofin opposite directions to a narrow range, one of said stops cooperatingwith a contact rigid with said arm to form an electric switch, and saidarm having thereon a magnet armature with oppositely facing surfacesadjacent said stops, and

a permanent magnet having poles disposed on opposite sides of saidarmature and having substantially parallel end faces each spaced fromsaid armature a distance substantially greater than the range ofmovement of said arm, said stops and pole faces being so spaced relativeto each other that said armature will be disposed on the magnetic centerof the gap between the pole faces when said arm is disposed equidistantfrom said stops.

7. An automatic control device having, in combination, an arm, meansresponsive to changes in the condition of the surrounding air for movingthe arm laterally back and forth, a pair of closely spaced stationarystops disposed on opposite sides of said arm and acting to limit themovement thereof in opposite directions to a narrow range, one of saidstops cooperating with a contact rigid with said arm to form an electricswitch, and said arm having thereon a magnet armature with oppositelyfacing surfaces adjacent said stops, and a permanent magnet having polesdisposed on opposite sides of said armature and having substantiallyparallel end faces each spaced from said armature a distancesubstantially greater than the range of movement of said arm, said polefaces being of equal areas so that the space center between the facescoincides with the magnetic center of the magnet, and said stopsand-pole faces being so spaced relative to each other that said armaturewill be disposed exactly midway between the pole faces when said arm isequidistant from said stops.

8. In an automatic control device of the general character described,the combination of a member mounted for movement in opposite directions,a sensitive control element arranged to move said member back and forthin response to changes in the condition of the surrounding air, stopmeans operating to limit the range of movement of the member in oppositedirections to a few thousandths of an inch, said member carrying meanscoacting with said stop means to form a control device and also carryinga magnet armature with oppositely facing surfaces, and a permanentmagnet having poles disposed on opposite sides of said armature with endfaces opposing said armature faces and each spaced from the adjacentarmature face in the limit positions of said member a distance severaltimes as great as the range of movement of the memher.

9. An automatic control device having, in combination, a member mountedfor movement back and forth, actuating means for said member adapted toapply a relatively small actuating force to said member for moving thesame in one direction or the other, stop means operating to limit themovement of said member in opposite directions within a range of a fewthousandths of an inch, said member carrying means coacting with saidstop means to form a control device and also carrying a magnet armaturewith oppositely'facing surfaces, and a permanent magnet having polesdisposed on opposite sides of said armature and having substantiallyparallel end faces spaced from said armature a distance such that thetotal nonmagnetic gap between the armature surfaces and said pole facesis at least several times as great as the total range of movement ofsaidmember.

DUNCAN J. STEWART.

